PEC Devices
The smart windows to which this invention is applicable typically have large surface areas and must be made at prices not too dissimilar from other specialist glazing materials used in the building industry. This is most difficult to achieve with complex PEC devices having the necessary clarity, whether darkened or bleached. Further, the PE and EC elements of a PEG window are inherently mismatched so that darkening tends to be non-uniform, partial and/or excessively slow.
A sun-sensitive smart window using a layer of liquid nematic crystals, controlled by external PE cells arranged to receive sunlight passing through the window, was disclosed by Mockovciak in U.S. Pat. No. 4,475,031. Sufficient PE cells were employed to provide the necessary voltage to darken the liquid crystal layer and a voltage regulator was used to ensure that the window was darkened according to the intensity of the incident light. However, a liquid crystal layer does not make a satisfactory electro-optical modulating material for a smart window as clarity is poor, particularly when the window is darkened. Also, since the PE cells and the voltage regulator must be installed separately from the window, the mounting and connection of this smart window will be expensive and unsightly. Others, such as Russell et al in U.S. Pat. No. 4,968,127 and Okaus in U.S. Pat. No. 6,015,086, have also disclosed similar self-powered sun-sensitive devices for use in spectacles
U.S. Pat. No. 5,384,653 to Benson discloses a double-glazed self-powered smart-window which employs a large-area EC element formed on a glass substrate, around the edge of which a plurality of silicon-based PE cells are formed by successive layer deposition. Sufficient PE cells are connected in series to provide the necessary switching voltage A selector switch is used to connect the EC cell to either the output of the PE cells or to an external power source such as a battery. A reversing switch is used to reverse the polarity of the selected power source to effect the darkening or bleaching of the EC cell. Although the device is essentially self-contained, the various layers required for the EC device and the PE cells are deposited under vacuum using masks by conventional PVD (physical vapour deposition), sputtering or CVD (chemical vapour deposition) techniques, severely limiting the area of window and making the device expensive. Also, the electrical arrangement necessitates the use of the same voltage for bleaching as for darkening which either results in excessively slow switching times or damage to the EC cell. Finally, the area of the window available to the PE cells is small so the power output from them is also small; moreover, the PE cells are unsightly. U.S. Pat. No. 5,457,564 to Levantis et al discloses an EC cell for use as a smart window, the cell having a pair of complementary electrochromic polymers plated onto respective transparent electrodes that are sputtered onto a pair of spaced glass substrates, the space being filled with an electrolyte The cell is powered by externally-located silicon-based PE cells arranged behind the device (with respect to the incident light). Light falling on the cells generates sufficient current to darken the electrochromic device and thereby reduce the light transmitted. Removal of the incident light allows the electrochromic device to discharge through the cells (which then act as forwardly-biased diodes), restoring the transparency of the device. Again, since the PE cells and the voltage regulator must be installed separately from the window, the mounting and connection of this smart window will be expensive and unsightly Also, electrical control of the window will not be satisfactory since the threshold voltage of silicon PE cells (acting as diodes) is a substantial proportion of the voltage needed to bleach the window. Bleaching will thus be partial and slow so that darkening will be far from proportional to incident light intensity over a diurnal cycle.
U.S. Pat. No. 5,377,037 to Branz et al, disclosed a PEC device for use with spectacle lenses comprising a self-regulating combination of a silicon-based PE cell and an EC cell each formed by the deposition of thin transparent layers on the lens surface. As about eight thin layers of transparent materials, including the electrolyte of the EC cell, must be laid down in succession by PVD, sputtering or CVD techniques on one glass substrate, the resultant multi-layer coating will be expensive and, not being protected by a second glass substrate will be delicate. These features make such a PEC device unsuited for use in smart windows. The PE cell of the Branz et al device is directly connected to EC cell, preferably via a bleed resistor, with the intention of making the PEC device self-darkening or `sun-sensitive`. However, the self-darkening function of the Branz PEC device will suffer from the same disadvantages as indicated in respect of the Levantis device. Further, since only one silicon PE cell is employed for each EC cell, the voltage will be insufficient to effect the rapid darkening necessary for spectacles, or even the more gradual darkening needed for smart windows.
In a Letter to Nature [Vol. 383, Oct. 17, 1996], Bechinger et al report the development of a PEG cell in which a photovolatic film forms one electrode and an electrochromic film forms the other electrode of the cell. The PE element used was the RPEC cell disclosed in the international patent applications WO 91/16719 and WO 96/08022 by Graetzel et al which employs a transparent dyesensitised nano-crystalline TiO.sub.2 semiconductor photoelectrode, a Pt coated counter electrode and a liquid electrolyte arranged there-between The EC element used was based upon the conventional WO.sub.3 -Li.sup.+ system using a liquid polymer electrolyte containing the Li.sup.+ ions. When such a PEC cell is exposed to light in an open-circuit condition (ie, without its electrodes being connected together), the voltage developed by the PE element does not cause darkening of the cell (as no current flows and Li ions are not transported to the working electrode of the EC element). When the exposed cell is short-circuited, current flows to carry the Li ions to the working electrode and darken the cell. Though the reported experiments were with small area cells (1 cm.sup.2 and 25 cm.sup.2), spatially localised darkening was noticeable. It was suggested that this would be an advantage where the cells were to be used for display purposes as they could be written-to using a laser beam or the like. However, this property is a serious disadvantage for large area smart windows. Moreover, the range and speed of darkening and bleaching control available by simply short-circuiting and open-circuiting the cell is limited due to the very low voltage/current available from the single RPEC cell.
Charge Control in EC Smart Windows
EC smart windows are inherently slow to switch between darkened and bleached states but great care must be taken in attempting to speed up the transition by increasing the charge rate and amount of charge delivered:
The working life (the number of cycles) of an EC cell will be seriously reduced if it is charged or discharged too fast or too much; PA1 Because the rate of darkening/bleaching is diffusion-limited, excessive localised charge concentrations can occur with resulting damage. PA1 Because the charging voltage is normally applied via a very thin TCO (transparent conducting oxide) film, the voltage is distributed unevenly over the film, again leading to the danger of excessive localised charge concentrations. PA1 As a CE cell ages its charging characteristics change, so a charge (or charge rate) which was safe when the cell was new will no longer be safe when it is old.
It is known to use integration techniques to keep track of the charge-state of an EC cell when it is being incrementally charged and discharged so that the danger of excessive charging/discharging is reduced. U.S. Pat. No. 4,512,637 and 4,529,275 to Carl-Zeiss disclose the use constant current charging and a digital counter to keep track of the charge-state of the cell (through successive full or partial charging and/or discharging actions) so that the danger of total overcharging is mitigated. U.S. Pat. No. 6,365,365 to Saint Gobain discloses the use of a reference capacitor which serves as an analogue of the cell and is charged and discharged with the cell. When an incremental change in colouration (darkening) is required, the desired colouration is `dialled-in` as a reference voltage which is compared with the voltage on the capacitor. The cell and capacitor are then discharged or charged (as required) to bring the capacitor voltage to the reference voltage. This method has the advantage that self-bleaching (or discharge) of the cell can be compensated during intervals between incremental colouration or bleaching. However, neither the Carl-Zeiss nor the Saint Gobain disclosure is concerned with ensuring optimum charge-rates, not can they compensate for the aging of EC cells.
In our prior international patent application WO 98/16870, we disclosed a method of controlling an EC cell which adjusted charge rates to mitigate the danger of cell damage without an excessive trade-off in switching time. To charge (darken) an EC cell, a constant current is applied until a predetermined maximum voltage is reached, after which charging proceeds at constant voltage until either a maximum safe charge (in Coulombs) has been delivered or the charge rate (in Amperes) falls below a predetermined level. Since the maximum voltage will be reached earlier with old cells than with new cells, the method provides a degree of compensation for cell aging, A recent international patent application, WO 97/28484 by Pilkington PLC, disclosed a similar approach except that charging is continued at constant voltage for a predetermined time after the maximum voltage limit is reached.
However, these methods switch the cells more slowly then necessary because. (i) the initial constant-current limit must be set conservatively, (ii) this limit is--inappropriately--the same for new and old cells and (iii) the charge rate is reduced well below the safe level during the initial stages of the constant voltage regime.